1. Field of the Invention
This invention pertains to methods for recovering metals, such as nickel and cobalt, from metal-rich soil using phytoextracting or phytomining techniques. Metals can be selectively extracted from soil by cultivating certain metal hyperaccumulating plants, such as Alyssum plants, on soil treated to adjust the pH.
2. Related Art
Industrial practices such as mining, smelting and disposing of manufacturing wastes have increased the concentrations of toxic metals in the environment. For example, at many nickel mining and smelting sites, levels of nickel and cobalt in soil have become so high that few plants survive, resulting in severe disruption of local ecosystems. Once nickel and cobalt enter soil, their removal is difficult since they are relatively immobile and they do not degrade into less toxic substances. The size of the areas affected by smelter and mine wastes are usually so large that engineering methods of soil remediation, such as soil removal and replacement, are too expensive to be practical (Cunningham et al., “Phytoremediation of Contaminated Soils,” Trends Biotechnol. 13: 393-397 (1995)).
The ability of certain plants to grow in metal-containing or metal-contaminated soil, and to actively accumulate heavy metals in their tissues, has created an interest in using such plants to extract metals from soil. Growing plants, including crops, on contaminated soil to extract contaminants is referred to as phytoextraction. This method is particularly effective in arable contaminated soils because it causes little disruption or dispersal, while preserving soil fertility and landscapes.
Nickel is one of the most widely found, and technologically important metals. It is a natural constituent in all soils, being particularly high in concentration in certain types of soil and geological materials such as serpentine, lateritic serpentine, ultramafic and meteor-derived soils. Cobalt, another valuable metal, has chemical and geological characteristics very similar to nickel and is generally found in the same soils. Other metals that may be found in such soils include those of the platinum and palladium families such as palladium, rhodium, ruthenium, platinum, iridium, osmium and rhenium, and metals such as selenium, zinc and cadmium.
Sites containing serpentine, lateritic serpentine, ultramafic and meteor-derived soils and materials can be conventionally mined or cultivated with metal-accumulating plants. Using such plants to extract metals from mineralized (geogenic) soils is referred to as phytomining.
U.S. Pat. No. 5,364,451 to Raskin et al., is directed to a method of remediating polluted soils at a reduced cost. Raskin et al. remove metals from metal-rich soil by growing plants of the family Brassicaceae in the metal-rich soil. While Raskin et al. generally describe a variety of plants and a large number of metals that may be recovered, the examples mainly describe the recovery of chromium and lead from genetically altered plants. Thus, although promising, Raskin et al. offer little basis for an opportunity to proceed directly with soil phytomining or phytoextraction through plant growth or cultivation.
U.S. Pat. No. 5,785,735 to Raskin et al., is also directed to methods of remediating polluted soils. Raskin et al. remove metals from metal-rich soil by growing crop and crop-related members of the plant family Brassicaceae in the metal-rich soil. The methods require the formation of a complex between the metal and a chelating agent added to the soil, the application of an electric field to the soil or a reduction in the pH of the soil. While Raskin et al. generally describe a variety of plants, the specification mainly describes the recovery of metals from genetically altered plants. Thus, again, Raskin et al. offer little basis for an opportunity to proceed directly with soil phytomining or phytoextraction through plant growth or cultivation.
Scientists recognize that increasing the pH of soil decreases the ability of farm crops to take-up heavy metals. U.S. Pat. No. 5,711,784 to Chaney et al. reflects the belief in the art that reducing the pH of the soil “increases the phytoavailability of nickel and cobalt.” As disclosed by Chaney et al., a “reduced pH increases solubility, and optimizes the release of these metals for absorption by the roots and translocation to the above-ground tissues of the plant.” However, reducing the pH of the soil also renders the metals more mobile and may allow for further contamination of the site. Therefore, cultivating plants which are hyperaccumulators of nickel, cobalt and other metals through phytoextraction or phytomining, is a desirable alternative as a means for recovering such metals.